As environmental problems such as climate change have recently emerged and the exhaustion of petroleum resources is expected to be depleted, attempts to produce most compounds by biotechnology as an alternative to chemical processes have been made, and the need for the development of related technology has increased. In addition, because of high petroleum prices, the production of compounds by microbial metabolisms and biological processes is becoming more price-competitive with petrochemical processes. Thus, the demand for the development of microbial strains and fermentation techniques that can solve energy problems is highly increasing. Among various biochemical compounds, lactic acid can be used in various applications, including a monomer for polylactic acid (PLA) that is a biodegradable polymer, a food additive, a precursor for drugs, etc., and has been of increasing interest.
Microorganisms reported to be used for the biological production of lactic acid (Appl. Microbiol. Biotechnol., 45, 307, 1996; Enzyme Microb. Technol., 26, 87, 2000; Korean Patent Application No. 10-2003-0090204) include lactic acid bacteria of the genus Lactobacillus and the genus Lactococcus, fungi of the Rhizopus (Appl. Biochem. Biotechnol., 51, 57, 1995; J. Biosci. Bioeng., 97, 19, 2004), yeasts of the genus Saccharomyces (Appl. Environ. Microbiol., 71, 2789, 2005), and E. coli. Lactic acid bacteria have a shortcoming in that the lactic acid produced has low optical purity, because it is a mixture of D- and L-forms. Fungi have a shortcoming in that large amounts of byproducts such as glycerol and ethanol are produced. Yeasts have shortcomings in that a large amount of ethanol is produced as a byproduct and in that lactic acid is produced in low yield.
In connection with the production of lactic acid in E. coli, it was reported that a mutant strain with a deletion of alcohol dehydrogenase (adhE) and phosphotrans acetylase (pta) genes produces lactic acid (J. Bacteriol. 171, 3650, 1989). In addition, there are known a method of using a mutant strain having a deletion of alcohol dehydrogenase (adhE) gene and phosphoenolpyruvate carboxylase (ppc) genes (Appl. Environ. Microbiol., 65, 1384, 1999); Korean Patent Application No. 1994-0004034), and a method of using pyruvate formate-lyase (pfl), fumarate reductase (frdABCD), alcohol dehydrogenase (adhE) and acetate kinase (ack) genes. Also, it is known that a mutant microorganism with a ubiquinone biosynthetic gene mutation accumulates lactic acid (Appl. Environ. Microbiol., 69, 399, 2003). Moreover, it was reported that deletion of the fnr gene (encoding a transcriptional regulator) and arcBA genes (encoding two-component response regulators) of E. coli leads to an increase in the production of lactic acid (Biochemical Engineering J. 42, 229-236, 2008), and the analysis of the metabolic carbon flux in mutant strains having a deletion of one or more of these genes indicated that the production of lactic acid increased (Metabolic engineering 8, 619-627, 2006). Known methods related to the production of lactic acid in E. coli are mostly performed using improved fermentation processes and mutant strains that have a deletion of the above-described known genes.
Currently, with the rapid exploitation of microbial genome information through new next-generation sequencing technology and the development of omics technology, platform technology capable of investigating microbial physiology and metabolism at the system level is being provided, but the functions and interactions of microbial genes have not yet been sufficiently elucidated. Organic acids and ethanol, which are produced during microbial fermentation which involves electron transfer and rearrangement, and this fermentation process is performed while maintaining the intracellular redox balance. In this process, oxidoreductases play an important role. A large number of oxidoreductases are present in the microbial genome, and networks regarding the connection between oxidoreductases are not sufficiently known.
The present inventors have found that enzymatic genes controlling the redox balance in an unknown respiratory system can control the metabolic flux of carbon by controlling the redox balance, and have identified genes playing an important role in the production of lactic acid, succinic acid or ethanol in the metabolic flux of carbon. Accordingly, the present inventors have made efforts to develop strains that highly produce lactic acid, succinic acid or ethanol, respectively, and as a result, have found that lactic acid, succinic acid or ethanol can be produced in high yield by preparing mutant strains wherein one or more of oxidoreductase genes and regulatory genes are inactivated or deleted, and culturing the prepared mutant strains. Based on this finding, the present invention has been completed.